FIG. 12 illustrates a radial cross section of a traditional permanent magnet direct current (PMDC) motor 30. The PMDC motor comprises a stator and a rotor rotatably mounted to the stator. The stator comprises a housing 20 which comprises two flat side portions 22 and two curved connecting portions 24. The stator also comprises two magnets 26, which are fixed onto the inner surface of the two curved connecting portions, respectively. The rotor has a rotor core 28 confronting the two magnets 26.
There is a desire for a smaller motor having an equal or increased performance. In other words, there is a desire for a motor having higher power density. There are two ways to increase a motor's performance, one being to increase the number of magnet poles, the other being to increase the outer diameter of rotor core. However, both of these two methods will usually result in an increased motor size. In some applications, motor size is a very critical factor. For example, a window lift, which comprises a motor and a gearbox driven by the motor, is usually installed in a limited space formed by two parallel side walls of a car door, and a special motor size and motor shape is required. For example, referring to FIG. 12, the diameter D2 between the two flat side portions 22 should not larger than a predetermined size, since the distance between the two side walls of a car door is limited. Whereas the diameter D1 may be larger as it extends in a direction between the wall panels of the door.